In this paper we introduce a new application of time-resolved X-ray tomography (XRT) to imaging of the washcoating process in catalytic monolith filters with a honeycomb structure (DPF, GPF). The method is applied to several alumina washcoats with different particle size distributions deposited on a cordierite substrate. The results bring new insights into the drying dynamics of the porous filter wall as well as the on-wall catalyst layer, including the process of layer shrinking and cracking. The XRT results are supplemented by cross-section scanning electron microscopy images and mercury intrusion porosimetry of the final structures. In addition to the statistical analysis of the washcoat distribution, the impact of the washcoat structure on filter pressure loss is determined by experiments.